This invention relates to transitional means between different transmission lines and for the fabrication thereof. More particularly, the subject invention relates to a novel transitional device for certain interconnections associated with planar circuits, the device being especially adapted for making measurements in probing a circuit under testing condition without resorting to conventional transitional techniques.
Integrated circuits usually consist of a large number of active circuit electronic components that generate or amplify electromagnetic energy in combination with passive components that detect or control such energy upon a semiconductor substrate. Integrated circuits are often fabricated by a number of process steps including film formation, impurity doping, lithography and etching which, in turn, call for special techniques, including vacuum and vapor-phase deposition, including molecular beam epitaxy, solid-state diffusion, implantation, and sputtering.
Planar structures or electrical circuits are well-known to the art, particularly in the field of integrated circuit technology, and more particularly, in the field of microwave technology. As regards such planar electrical circuits, distributed components are generally dispersed along an entire length or area of the circuit instead of being concentrated or lumped in confined areas. More importantly, distributed components are generally constructed of a metallized strip of a certain configuration with a ground plane separated by a solid dielectric and are commonly referred to as microstrip, coplanar and slotline structures. The metallized strip is a flat strip-type conductor on a dielectric substrate and is disposed close to an r. f. ground in such a way as to provide particular r. f. impedance for the circuit.
Integrated circuits are devices with the various active and passive components arranged in an array or pattern upon a planar substrate. It is the planar configuration with the closely packed circuit components that are simultaneously fabricated directly thereon that make such devices so practical. Indeed, the planar structure is responsible for the ease of fabrication and the economic importance of integrated circuits, the substrate thereof being any one of several materials including alumina, Teflon laminated board as well as semiconductor crystalline materials.
There are several types of integrated circuits that are used commercially. One common type is the so-called monolithic microwave integrated circuit (MMIC). Briefly, a monolithic integrated circuit has all of the circuit components and elements formed in situ on the semiconductor substrate. In a monolithic integrated circuit, there are no components that stand in substantial relief on the planar substrate as found, say, in a hybrid integrated circuit, as the entire integrated circuit of the monolithic structure is formed in or on the substrate. Again, it is this flat or planar structure that allows batch processing and mass fabrication of reliable circuits.
Because of more and more circuit miniaturization, circuits have become exceedingly small in size with the various active and passive components being closely packet thereon. It can, therefore, be appreciated that such miniature circuits present special problems in attempts at testing, tuning or troubleshooting. In the fabrication of integrated circuits, a producer must depend upon computer-aided design techniques to achieve uniform quality of such integrated circuits. It is only recently that probing equipment has become commercially available for testing miniature circuits whereby reliable and accurate measurements can be made on an ongoing basis. Such measurements are necessary and desirable, especially before dicing or cutting a large array of the wafer structure to form the individual integrated circuits.
Conventional probe cards have been employed for making radio frequency measurements, especially at low frequencies for silicon integrated circuits. When probing is done on the whole wafer, this is known as on-wafer circuit testing. Probe cards are capable of operating at frequencies up to above 4 GHz and generally use needle-like probes. However, because of inductance of the individual probes and the associated inter-probe capacitances, such probe structures at higher frequencies present serious difficulties in making reliable measurements, particularly at frequencies much above 10 MHz.
Test probe heads have been used to perform r. f. measurements and are constructed and designed to provide low loss, closely matched (low imput voltage standing wave ratio) signal path from the measurement equipment to the r. f. input and output connecting points of the component being investigated on an integrated circuit. By the use of the testing probes heads, circuity of an electrical component or device can be readily evaluated during its early stages of modeling and, of course, during production, and any defective circuits readily identified.
In probing machines currently in use, a wafer or other electronic device to be tested is properly aligned and the tip contacts or electrodes of the probes are guided to selected pads (usually round or square shaped indicia or about two to five mils in size) on the device under test and make interface engagement of the tip contacts with the electrodes. Upon making interface contact, the probing equipment evaluates the electrical operation of the device under test.
Coplanar wave guide (CPW) probes have been utilized to make measurements at very high frequencies. Such CPW probing devices have been found to be operative at high frequencies in the complex and closely packed geometrical environment where many thousand of components are patterned on a substrate.
In making measurements on a planar structure such as an integrated circuit, the signal and ground electrodes of a probe must make contact and be in alignment with precise points on the component under test. Using a probe that employs a coplanar waveguide for contacting corresponding points on a planar structure is not a easy undertaking. In this regard, it should be appreciated that the probe contact is generally to an active device through a microstrip transmission line, a highly utilized metallization of integrated circuits.